Mining or excavation of rock including pyrite and related minerals, such as found with coal formations, can result in water pollution. Exposure of rock containing sulfide minerals such as pyrite (FeS2), marcasite (FeS2 (orthorhombic)), and pyrrhotite (Fe1-xS (x=0-0.2)), to the atmosphere, such as by production of coal mine tailings, can result in oxidation of the minerals to sulfuric acid and soluble iron species. Water draining from coal mining tailings and other sources including iron sulfide and related sulfide minerals is commonly referred to as “acid mine drainage” or “acid rock drainage.” Acid mine drainage can have high concentrations of iron species, sulfuric acid, as well as other dissolved metal species, including heavy metals such as molybdenum, tungsten, chromium, manganese, nickel, arsenic, vanadium and the like. The composition of the metal species contaminating the water can vary greatly, depending on the source of the water. Likewise, mines originally used for extracting minerals such as silver, gold and copper can also produce a subsequent water drainage containing the above-mentioned dissolved metals.
A number of processes have been employed for treating neutralized and/or otherwise treat mine discharge. For example, acid mine drainage can be treated with bases such as calcium carbonate, calcium oxide, calcium hydroxide, sodium hydroxide, sodium carbonate, and ammonia. In addition to chemical treatments, acid mine discharge has been treated using a variety of biological treatment schemes, employing biological mechanisms to neutralize the waste water and remove metals from solution. Biological mechanisms typically rely on the construction of containment ponds and the like to subject the acid mine discharge to extended biological action.
Treatment with bases may cause the ferrous and ferric ions to precipitate and form a very hydrous, gelatinous hydrated iron hydroxide (Fe(OH)x, x=2 and 3), which forms difficult to remove flocs.
A number of multi-step processes have been proposed to treat the acid mine drainage in a step-wise manner, frequently for the purpose of recovering specific metal species from the wastewater.
For example, U.S. Pat. No. 5,505,857 (“Misra”) discloses a three-step process for selectively recovering metals contained in wastewaters as metal precipitates and/or spinel ferrite and water suitable for discharge into the environment. In the first step, the pH is adjusted and a sulfur compound is added to precipitate at least one non-ferrous metal ion as the sulfide. In the second step, aluminum is removed by further adjusting the pH of contaminated water and adding a precipitant for the aluminum. In the final step, the pH is further adjusted to strongly alkaline and solution is oxidized, and spinel ferrite is precipitated and removed.
U.S. Pat. No. 5,645,730 (“Malachosky”) discloses treating acid wastewater containing heavy metals such as acid mine drainage with fly ash to reduce the level of sulfate ions. In one aspect of the process, solid silicates salts are added to avoid the formation of undesirable very hydrous iron precipitates.
Active and passive chemical technologies as well as various biological strategies for treating acid mine drainage are reviewed in D. B. Johnson et al., “Acid mine drainage remediation options: a review,” Science of the Total Environment 338 (2005) 3-14, incorporated herein by reference.
There is continuing need for a process for treatment of acid mine discharge, particularly for a process which can be used to treat large quantities of wastewater quickly.